The present invention relates to oxygen sensors suitable for use in spacecraft and for other uses. More specifically, it relates to shortening the break-in time for the useful life of oxygen sensors such as are used on the space shuttle.
It has been observed that an oxygen sensor as illustrated in FIG. 1 has a total life of approximately 8000 hours when exposed to 1 atmosphere of air at 75.degree. F. Total life includes both useful operating life and exposure to oxygen for testing, storage and for any other reason. The calculated total life of a sensor as illustrated in FIG. 1 is approximately 9200 hours. The original specifications of the National Aeronautics and Space Administration for such sensors is 6236 operational hours of useful operating life.
Because substantial initial break-in time as well as testing and operation of sensors is necessary prior to an actual mission, an appreciable portion of the total life of such a sensor is used up in the break-in period, in laboratory testing, in various calibrations and in an air exposure aboard the spacecraft before the scheduled launch. This can be further extended by delays in the actual launch thus using up a further portion of the total life of the sensor. In all, a significant fraction of the total life of the sensor is used in this way. For these and other reasons, reduced sensor output has mandated premature sensor replacement prior to a number of space shuttle missions. There is considerable interest in NASA in avoiding such operating complications in providing oxygen sensing in a spacecraft.
The term oxygen sensor as used herein includes an oxygen sensor cell such as is illustrated in FIG. 1 and electrical circuitry most of which is external to the cell and which is illustrated schematically in FIG. 2. An essential component of an oxygen sensor cell is a counter electrode one form of which is illustrated in FIG. 3.